This invention concerns ammonium ionomers, and a method for preparing them by contacting a polymer having a substantially fluorinated, but not perfluorinated, polyethylene backbone having pendant groups of fluoroalkoxy sulfonyl fluoride, with an excess of ammonium carbonate solution. This invention further concerns a method for forming ionomers by ion exchange with ammonium ionomers. The ionomers so formed are useful in electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes.
Copolymers of vinylidene fluoride (VDF) with vinyl alkoxy sulfonyl halides are known in the art.
The disclosures in Ezzell et al. (U.S. Pat. No. 4,940,525) encompass copolymers of VDF with vinyl ethoxy sulfonyl fluorides containing one ether linkage. Disclosed is a process for emulsion polymerization of tetrafluoroethylene (TFE) with the vinyl ethoxy comonomer.
Banerjee et al. (U.S. Pat. No. 5,672,438) disclose copolymers of VDF with vinyl alkoxy sulfonyl fluorides containing more than one ether linkage.
Connolly et al. (U.S. Pat. No. 3,282,875) disclose the terpolymer of VDF with perfluorosulfonyl fluoride ethoxy propyl vinyl ether (PSEPVE) and hexafluoropropylene (HFP). They broadly teach an emulsion polymerization process said to be applicable to copolymerization of vinyl ethers with any ethylenically unsaturated comonomer, with greatest applicability to fluorinated monomers.
Barnes et al. (U.S. Pat. No. 5,595,676) disclose xe2x80x9csubstantially fluorinatedxe2x80x9d copolymers of a vinyl ether cation exchange group-containing monomer with a xe2x80x9csubstantially fluorinatedxe2x80x9d alkene. The copolymer is produced by controlled addition of the alkene in emulsion polymerization, followed by hydrolysis in NaOH. PSEPVE/TFE copolymers are exemplified.
Hietala et al., J. Mater. Chem. Volume 7 pages 721-726, 1997, disclose a porous poly(vinylidene fluoride) on to which styrene is grafted by exposing the PVDF to irradiation. The styrene functionality is subsequently functionalized to sulfonic acid by exposure of the polymer to chlorosulfonic acid. The resultant acid polymer, in combination with water, provides a proton-conducting membrane.
Formation of ionomers and acid copolymers by hydrolysis of the sulfonyl fluoride functionality in copolymers of TFE and fluoro alkoxy sulfonyl fluorides is well known in the art. The art teaches exposure of the copolymer to strongly basic conditions.
See for example, Ezzell et al. U.S. Pat. No. 4,940,525, wherein is used 25 wt % NaOH(aq) for 16 hours at 80-90xc2x0 C.; Banerjee et al. U.S. Pat. No. 5,672,438, wherein is used 25 wt % NaOH for 16 hours at 90xc2x0 C., or, in the alternative, an aqueous solution of 6-20% alkali metal hydroxide and 5-40% polar organic liquid (e.g., DMSO) for 5 minutes at 50-100xc2x0 C.; Ezzell et al. U.S. Pat. No. 4,358,545 wherein is used 0.05N NaOH for 30 minutes for 50xc2x0 C.; Ezzell et al. U.S. Pat. No. 4,330,654, wherein is used 95% boiling ethanol for 30 minutes followed by addition of equal volume of 30% NaOH (aq) with heating continued for 1 hour; Marshall et al. EP 0345964 A1, wherein is used 32 wt % NaOH (aq) and methanol for 16 hours at 70xc2x0 C., or, in the alternative, an aqueous solution of 11 wt % KOH and 30 wt % DMSO for 1 hour at 90xc2x0 C.; and, Barnes et al. U.S. Pat. No. 5,595,676, wherein is used 20 wt % NaOH (aq) for 17 hours at 90xc2x0 C.
Because of its high dielectric constant, high electrochemical stability, and desirable swelling properties, poly(vinylidene fluoride) is known in the art of lithium batteries as a highly desirable material for use as a membrane separator. For example Gozdz et al. (U.S. Pat. No. 5,418,091) disclose porous PVDF homopolymer and copolymer containing solutions of lithium carbonates in aprotic solvents useful as separators in lithium batteries.
Porous membranes of the type described by Gozdz, however, conduct both the cation and the anion back and forth across the separator, and are thus subject to concentration polarization during use, which degrades the performance of the battery in which it is used. So-called single ion conducting polymeric membranes, wherein the ionic carbonate is attached to the polymer backbone, thereby immobilizing either the cation or the anion, offer a solution to the concentration polarization problem, and are known in the art. One particularly well-known such single ion conducting polymer is Nafion(copyright) Perfluoroionomer Resin and Membranes available from DuPont, Wilmington, Del. Nafion is a copolymer of TFE and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) which has been hydrolyzed by treatment with an alkali metal hydroxide according to the teachings of the art as hereinabove described.
It is further known in the art, and hereinbelow shown, that PVDF homopolymers and copolymers are subject to attack by strong bases such as the alkali metal hydroxides taught in the art hereinabove cited. Of particular importance is that the attack of basic nucleophiles on a copolymer of VDF and perfluorovinyl ethers results in the removal of the vinyl ether moiety from the polymer, see W. W. Schmiegel in Die Angewandte Makromolekulare Chemie, 76/77 pp 39ff, 1979. Since the highly preferred monomer species taught in the art, and exemplified by DuPont""s Nafion and similar products, for imparting ionomeric character to various polymers is a vinyl ether terminated by a sulfonyl halide functionality, the sensitivity to base attack of the VDF copolymer formed therewith has prevented the development of a single-ion conducting ionomer based upon VDF. There simply is no means taught in the art for making the ionomer.
Doyle et al, U.S. Pat. No. 6,025,092, discloses ionomers formed with vinylidene fluoride copolymers by subjecting sulfonyl fluoride containing precursors to hydrolysis with alkali and alkaline earth metal carbonates, such as lithium carbonate, under mildly basic conditions. The method of Doyle et al, however is limited in that any excess over stoichiometric amounts of the hydrolyzing agent results in attack on the VDF backbone, causing polymer degradation. Thus the method of Doyle et al is limited in industrial applicability.
Barton et al, WO 0052085A1, discloses melt processible compositions comprising alkali metal ionomers having vinylidene fluoride monomer units, and liquids imbibed therewithin. The ionomers of Barton et al are not melt processible without incorporation of the liquids.
The present invention provides for an ionomer comprising monomer units of vinylidene fluoride and 0.5-50 mole % of a perfluoroalkenyl monomer having a pendant group of the formula
xe2x80x94(Oxe2x80x94CF2CFR)aOxe2x80x94CF2(CFRxe2x80x2)bSO3xe2x88x92NH4+
wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6.
The present invention also provides for a process for forming an ionomer comprising
contacting a polymer comprising
monomer units of vinylidene fluoride and 0.5-50 mole % of a perfluoroalkenyl monomer having a pendant group of the formula
xe2x80x94(Oxe2x80x94CF2CFR)aOxe2x80x94CF2(CFRxe2x80x2)bSO2F
xe2x80x83wherein R and Rxe2x80x2 are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6,
with an excess of a solution of ammonium carbonate for a period of time sufficient to obtain the degree of conversion desired to the ammonium sulfonate form of the polymer.
For the purposes of description in the present invention, the generic term xe2x80x9cionomerxe2x80x9d will be taken to encompass the ammonium sulfonate and the sulfonic acid forms of the polymer of the invention, as well as the alkali and alkaline earth salts thereof. For the purpose of the present invention, the term xe2x80x9cexcessxe2x80x9d when applied to the ammonium carbonate solution of the present invention means that the solution contains more, preferably many fold, more than the amount of ammonium carbonate necessary to achieve complete hydrolysis of the sulfonyl fluoride to the sulfonate based upon reaction stoichiometry. That is, xe2x80x9cexcessxe2x80x9d means beyond, preferably many fold beyond, the stoichiometric amount.
The term xe2x80x9csubstantially fluorinatedxe2x80x9d means that at least 50 mole % of the hydrogens of the corresponding polyethylene backbone have been replaced by fluorines.
In one aspect of the present invention the sulfonyl fluoride-containing precursor polymer is contacted with a many fold excess of ammonium carbonate solution, effecting the hydrolysis of the sulfonyl fluoride to the ammonium sulfonate without degradation of the polymer backbone. In another aspect of the present invention, the ammonium sulfonate ionomer may be melt processed, such as by thermal consolidation of ammonium sulfonate ionomer of the invention into a shaped article such as a polymer film, without the addition of any liquid to the polymer.
Means for forming the ammonium sulfonate ionomer into a film, sheet or other shaped article include melt pressing and extrusion using a screw extruder. Other means include roll milling and such other means well-known in the art of plastics processing for forming shaped articles of thermoplastic polymers. The ammonium sulfonate ionomer of the invention can also be formed into shaped articles according to solution methods disclosed in the art such as by dissolution in a solvent followed by solution casting of a film or sheet upon a substrate. However melt processing is preferred.
In an alternative embodiment of the present invention, the sulfonyl fluoride form of the polymer is first melt-formed into a sheet and then contacted with an excess of ammonium carbonate solution to effect hydrolysis to the ammonium sulfonate ionomer.
In a further embodiment, the ammonium sulfonate ionomer is contacted with a mineral acid, preferably an aqueous mineral acid, such as nitric acid, to form the sulfonic acid ionomer which is useful in fuel cells. In yet a further embodiment of the invention, the sulfonic acid ionomer is contacted with a solution, preferably an aqueous solution, of an alkali metal salt, such as LiCl, to form the alkali sulfonate ionomer useful in various electrochemical cells such as lithium batteries.
In a further embodiment, the ammonium ionomer may be contacted with a solution, preferably an aqueous solution, of an alkali metal salt such as LiCl to form the alkali metal ionomer by ion exchange. It is preferred, however, to first form the sulfonic acid followed by ion exchange to form the alkali metal, preferably the lithium, ionomer.
In all said foregoing embodiments, it is preferred that the ionomer undergoing the ion exchange processes be in the form of a film or sheet.
In the process of the invention vinylidene fluoride (VDF) is copolymerized with a non-ionic monomer (I) represented by the formula
CF2xe2x95x90CFxe2x80x94(Oxe2x80x94CF2CFR)aOxe2x80x94CF2(CFRxe2x80x2)bSO2Fxe2x80x83xe2x80x83(I)
where R and Rxe2x80x2 are independently selected from F, Cl or a fluorinated, preferably perfluorinated, alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6. Preferably R is trifluoromethyl, Rxe2x80x2 is F, a=1 and b=1. In the process of the invention, the copolymer so formed is contacted with an excess of a solution of ammonium carbonate to form an ionomer comprising monomer units of VDF and 0.5-50 mole %, preferably 0.5-36 mole %, of an ionic perfluoroalkenyl monomer having a pendant group of the formula
xe2x80x94(Oxe2x80x94CF2CFR)aOxe2x80x94CF2(CFRxe2x80x2)bSO3xe2x88x92NH4+
where R and Rxe2x80x2 are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6. Preferably, R is trifluoromethyl, Rxe2x80x2 is F, a=0 or 1, b=1.
The ammonium carbonate solution suitable for use in the present invention is a solution formed by adding ammonium carbonate to water, alcohol, organic carbonate, or mixtures thereof. Suitable alcohols include but are not limited to methanol, ethanol and butanol. Suitable carbonates include but are not limited to ethylene carbonate and propylene carbonate. Preferably the ammonium carbonate is dissolved in a mixture of methanol and water.
A preferred hydrolysis process of the invention comprises contacting the sulfonyl fluoride-containing polymer with an excess of a solution of ammonium carbonate in methanol (optionally containing another solvent such as water), in the range of ca. 0-85xc2x0 C., preferably room temperature to 65xc2x0 C. for a sufficient length of time to convert the desired percentage of sulfonyl fluoride to ammonium sulfonate.
Generally preferred are the mildest hydrolysis conditions possible consistent with timely conversion of the sulfonyl fluoride. The severe hydrolysis conditions taught in the art for hydrolyzing sulfonyl fluoride to sulfonate in the case of ionomers which do not include VDF, cause degradation of the VDF-containing copolymer in the present invention. The degree of conversion can be conveniently monitored by the disappearance of the characteristic infrared absorption band for the sulfonyl fluoride group at about 1462 cmxe2x88x921. Alternatively, 19F NMR spectroscopy may be used as described in the examples.
The ionomers prepared by the process of the invention include copolymer compositions in which the ionic monomer unit is present in the ionomer of the invention at concentrations ranging from 0.5 to 50 mole %, preferably 0.5-36 mole %.
Other cationic forms of the ion-exchange membrane can be achieved using ion-exchange procedures commonly known in the art and as outlined herein above (see for example Ion Exchange by F. Helfferich, McGraw Hill, New York 1962). For example, the protonic form of the membrane is preferably obtained by immersing the ammonium-ionomer into an aqueous acid.
Silver and copper sulfonate ionomers can be made by ion exchange with the ammonium sulfonate form of the polymer. For example, repeated treatment of the ammonium sulfonate ionomer with an aqueous solution of a silver salt such as silver fluoride or silver perchlorate would produce at least a partially cation exchanged silver sulfonate ionomer. In a similar fashion, the cuprous sulfonate ionomer can be produced by repeated treatment of the ammonium sulfonate ionomer with an aqueous acidic solution of a copper salt such as cuprous chloride.
In many applications, the ionomer is preferably formed into a film or sheet. Films of the ionomer may be formed according to processes known in the art. In one embodiment, the thermoplastic sulfonyl fluoride precursor is extrusion melt cast onto a cooled surface such as a rotating drum or roll, whence it is subject to hydrolysis according to the process of the invention. In a second embodiment, a sulfonyl fluoride-containing polymer is dissolved in a solvent, the solution cast onto a smooth surface such as a glass plate using a doctor knife or other device known in the art to assist in depositing films on a substrate, and the resultant film subject to hydrolysis according to the process of the invention. In a third embodiment, the sulfonyl fluoride copolymer resin is subject to hydrolysis by dissolution or suspension in a hydrolyzing medium, followed by optional addition of cosolvent, and filtration or centifugation of the resulting mixture, and finally solvent casting of the ionomer solution onto a substrate using a doctor knife or other device known in the art to assist in depositing films on a substrate.
In an alternative embodiment, it is found in the practice of the present invention that the ammonium ionomer is particularly amenable to melt forming. Thus, the ammonium ionomer may be isolated in the form of a powder, and the powder melt formed into a film or sheet which may then be subject to ion exchange according to the methods taught herein.
The ionomers prepared according to the practice of the invention may be terpolymers. Suitable third monomers include tetrafluoroethylene, chlorotrifluoroethylene, ethylene, hexafluoropropylene, trifluoroethylene, vinyl fluoride, vinyl chloride, vinylidene chloride, perfluoroalkylvinyl ethers of the formula CF2xe2x95x90CFORf where Rf=CF3, C2F5 or C3F6. Preferred termonomers include tetrafluoroethylene, hexafluoropropylene, ethylene and the perfluoroalkylvinyl ethers. Termonomers are preferably present in the polymer at a concentration of up to 30 mole %.